Phototransistors. My personal preference for Block 1 is a phototransistor. It's not as quick as a photodiode, but it offers some gain. Also, a phototransistor is more durable and easier to work with than a solar cell. See the phototransistor circuits nearby. You can use many types of phototransistors in these circuits. Two that I've used recently are... Texas Instruments TIL414 from RadioShack Ledtech LT9593-91-0125 from All Electronics

High-gain phototransistor circuit for mechanical video pickup

Requirement: Clear window. Make sure that you use a phototransistor with a clear, plastic window or case. Types with black windows or envelopes won't work for this project. A black case blocks visible light. Some infrared phototransistors have these black (or dark color) cases. Other devices have a clear case, and respond to both infrared and visible light. These dual-purpose types are fine. Radio Shack, Mouser, Digi-Key and All Electronics carry suitable phototransistors.

Circuit with faster response speed, but voltage gain of 0.9

Install the phototransistor in a box with a window. The window should be about the size of our picture frame, or slightly larger. Mount the phototransistor about two inches behind the window. The window and phototransistor must face the Nipkow disc apertures.

How do we design a phototransistor preamplifier? We start by examining circuits. Maybe we can use or modify something that already exists. There are lots of circuits, and many are easy to build. Below is one idea that might help...

Low-level preamp. Next, we add a low-level preamplifier. Our preamp Q2 is an NPN, common-emitter stage. Our phototransistor is Q1. The Q1 circuit is an emitter-follower stage. We direct couple this stage to Q2. Output signal Vo comes off Q2 between collector and ground. Transistor Q2 is a general-purpose, silicon, small-signal type. For example, 2N2222, 2N3904 or 2N4401.

Low-level preamplifier improves phototransistor sensitivity

(Hfe: 200 or above. Germanium types won't work in this circuit.) Limits. The signal is still a high-impedance one. Yet we have a much stronger signal than we started with. At 1 kHz, Q2's unloaded gain is 179. Unfortunately, this is a pie-in-the-sky gain figure. When we connect Q2 to anything meaningful, our gain crashes back toward earth. Capacitors. You'll notice the large decoupling capacitor. We need Paul Bunyonesque capacitors so that we can reproduce frequencies down to nearly DC. These frequencies account for crucial large picture details.

Need: More Gain! Our new circuit is helpful for experiments. Yet we probably still need more voltage gain. I suspect that our solution requires two or three more transistors.

Here's an easy way to beef up that amplifier: "Tinkertoy" stages that you just snap together. Perfboard construction is fine for this circuit. Notice that NPN and PNP stages alternate. For the PNP device, use a 2N3906, 2N2907 or compatible device. Again, Hfe (beta) must be 200 or better, and the device must be silicon. You can add one, two, or several stages this way. Just follow each NPN transistor with a PNP stage. Then follow each PNP stage with an NPN stage. You'll need an LED driver to test the circuit with. Here's the LED driver circuit.

Easy, snap-together stages give us the gain we need

Here's the design procedure...

1. Add one transistor stage at a time.

2. Then test the circuit revision with an LED driver. The only reliable test is a scanner test. Scan the LED and watch for some sort of image. The image doesn't have to be perfect. It must have contrasting areas, though. Reflected light falling on the phototransistor must cause the LED to flicker.

3. As necessary, add stages one by one.

4. After adding each stage, retest.

5. When you get enough gain, stop adding stages.

6. If you don't succeed after adding five stages, you have a wiring error. Quit and troubleshoot the circuit.

How do you know when you have enough gain? Easy. Plug the last stage into the input of the LED driver. Do the LEDs twinkle when they pick up reflected (not direct) light? Do the LEDs twinkle this way when you scan them? Good. Then you probably have the gain you need. Otherwise, repeat the design procedure.

Be sure to wire the stages correctly. Before switching on the power, check everything. A visual check takes far less time and costs less money than a smoke test! Make sure that you haven't reversed the PNP and NPN stages. You can damage some transistor types by reversing the collector and emitter connections.

Picture polarity. Some camera circuits product positive-polarity signals. The second schematic above is an example of this type circuit. Other circuits invert the polarity and produce negative signals. For example, see the first schematic above.

Polarity effects. For our purposes, signal polarity is only important in the resulting picture. If we reproduce negative signals on the TV screen, the light and dark shades appear reversed. (In color TV, the colors might also reverse. That is, red appears as cyan, green becomes magenta, and blue becomes yellow.) Obviously the effect is totally unnatural.

Restoring positive polarity. We can easily restore a negative picture to its original polarity values. The trick is to run the signal through an inverting amplifier. At the output, a negative signal again becomes positive.